Many telecommunications products are required to conform to a publicly known standard or protocol. For example, CCITT Recommendation G.726-1990 specifies how a digital telephone signal is to be compressed before transmission, and how a received digital signal is to be expanded after reception, using a well-known technique known as adaptive differential pulse code modulation (ADPCM). The G.726 standard specifies the functionality that is required for the receive (ADPCM decoder) and transmit (ADPCM encoder) signal processing functions. G.726 allows for the conversion of a 64 kilobit-per-second (kbps) pulse code modulation (PCM) channel to and from a 40, 32, 24, or 16 kbps ADPCM channel. G.726 incorporates the previously-existing G.721 (32 kbps) and G.723 (24 kbps) standards.
The telephone signal sampling rate is relatively slow compared to digital logic speeds now available. The relative slowness allows a data processor to implement the G.721 protocol through microcode. In a typical implementation, for example the Motorola MC145532 ADPCM Transcoder, a special type of data processor known as a digital signal processor (DSP) is used. A DSP is a data processor with an instruction set designed to optimize performance in real-time digital signal processing applications, and DSP architectures are known in the art. By being optimized for signal processing, however, the DSP eliminates many of the instructions available from a complex instruction set (CISC) data processor.
Some telecommunications standards incorporate other standards. The U.K. Cordless Telephone, Second Generation (CT-2) specifies the use of G.721 ADPCM encoding and decoding functions. A CT-2 handset receives an analog voice signal from a microphone. The voice signal is converted to PCM, processed through a G.721 ADPCM encoder, and then modulated and transmitted (along with other signalling information) according to the CT-2 Common Air Interface (CAI) protocol to a base station. Transmit and receive signals are sent and received to and from the base station in packets in a pingpong fashion. For signal reception, a packet is received, demodulated, processed through a G.721 ADPCM decoder, converted from digital PCM to analog, and then provided to a speaker.
It is also desirable to have a volume control which can adjust the signal provided to the speaker within a wide range and with a fine resolution. Conventional volume controls, such as potentiometers, directly attenuate the receive analog data stream. These volume controls are costly in terms of space and can be inaccurate, however. In order to reduce handset cost and improve performance, new ways to adjust the volume are required.